1. Field of the Invention
The present invention relates to a power supply, in particular to a switched mode power supply (SMPS) and a method of controlling the same.
2. Description of the Background Art
In a conventional switched mode power supply (SMPS), when the electrical loads connected with an output terminal of the switched mode power supply do not draw electric power, the switched mode power supply assumes a standby state. Only is when the electric loads require power, the SMPS provides power to the electric loads, by operating in a normal mode.
FIG. 1 illustrates a circuit of a switched mode power supply which is composed of a main power unit, an auxiliary power unit and a controller.
The main power unit includes an input filter unit 101 removing noise from AC power supplied from an AC power source, a rectification/smoothing unit 102 rectifying the AC power to DC by a full wave rectification and smoothing the DC voltage to a certain level, a main transformer 104 to a primary winding of which is repeatedly supplied and disconnected DC power supplied from the rectification/smoothing unit 102 in accordance with an ON/OFF operation of a switching device Q1, a snubber 103 removing noise from power supplied to the primary winding of transformer 104, the noise generated when the switching device Q1 is operated, a rectification/smoothing unit 105 rectifying AC voltages induced in the secondary windings of the main transformer 104 and smoothing the resulting DC voltage to a certain level, a rectification/smoothing unit 105-1 generating a certain direct current voltage from the AC voltage induced in an auxiliary winding of the main transformer 104, and a driving controller 106 receiving an output voltage of the rectification/smoothing unit 102 and the direct current voltage from the rectification/smoothing unit 105-1 and controlling an ON/OFF operation of the switching device Q1 in accordance with a control signal from a signal feedback unit 107, which will be explained later.
The auxiliary power unit includes a driving controller 111 receiving an output voltage of the rectification/smoothing unit 102 and controlling an ON/OFF operation of a switching device Q2, and an auxiliary transformer 108 to a primary winding of which is repeatedly supplied and disconnected the output voltage of the rectification/smoothing unit 102 in accordance with the ON/OFF operation of the switching device Q2 thereby inducing an AC voltage in a secondary winding of the auxiliary transformer 108 which is connected to a rectification/smoothing unit 109 and a rectification/smoothing unit 109-1. The rectification/smoothing unit 109-1 rectifies the AC voltage induced in the secondary winding of auxiliary transformer 108 and outputs a certain direct current voltage to the driving controller 111 in accordance with the switching operation of the switching device Q2. The rectification/smoothing unit 109 rectifies the voltage induced in the secondary winding of the auxiliary transformer 108 in accordance with a switching operation of the switching device Q2 and smoothes the thusly rectified a direct current voltage. The auxiliary power unit also includes a snubber 110 removing noise which is generated in the auxiliary transformer 108 when the switching device Q2 is operated.
The control unit includes a microcomputer 114 receiving as its supply power in a normal operating mode an output voltage among the output voltages outputted from the rectification/smoothing unit 105, and also receiving an output voltage of the rectification unit 109 as a supply voltage in a power saving mode. The microcomputer 114 outputs a control signal for controlling an ON/OFF operation of the switching device Q1 in the normal mode. The control unit also includes a signal feed back unit 107 detecting information on the state of the power consumption of loads connected to the output terminals of the rectification smoothing unit 105 receiving a control signal from the microcomputer 11410 and transferring the information and the control signal to the driving controller 106, a timer 112 generating a timing signal and supplying the thusly generated timing signal to the microcomputer 114, and a backup unit 113 receiving a direct current voltage from the rectification/smoothing unit 105 and also from the rectification/smoothing unit 109 and storing backup power for operating of the microcomputer 114 in the standby mode.
As shown in FIG. 2 illustrating in detail the circuit of FIG. 1, the rectification/smoothing units 105, 105-1, 109 and 109-1 are circuits each formed of a diode D1-DM, DD5-DD6 and a capacitor C1-CM, CC5-CC6. The rectification/smoothing unit 109 further includes a regulator 109-2 and a diode DD7.
The operation of the thusly constituted conventional switched mode power supply will be explained.
When AC mains power is applied into the input filter unit 101, the input filter unit 101 removes noise from the AC mains power and outputs it to the rectification/smoothing unit 102. The rectification/smoothing unit 102 rectifies the noise-removed AC power by a full wave rectification, smoothes it, and then outputs a certain level direct current voltage to the primary windings of the transformers 104 and 108.
At this time, as the driving controller 106 in the main power unit turns on/off the switching device Q1 on/off, AC voltages are induced in the secondary windings of the main transformer 104 and are applied to the rectification/smoothing unit 105. As the driving controller 111 in the auxiliary power unit turns the switching device Q2 on/off, an AC voltage is induced in the secondary winding of the auxiliary transformer 108 and is applied to the rectification/smoothing unit 109.
The rectification/smoothing unit 105 outputs the DC voltages which are smoothed after a half wave rectification to the loads (not shown), and the rectification/smoothing unit 109 outputs a regulated voltage, which is smoothed after a half wave rectification through the regulator circuit 109-2 and the diode DD7. At this time, the regulated voltage outputted from the rectification/smoothing unit 109 is supplied to the back-up unit 113 and the microcomputer 114 via a line connected with diode DD7.
In the normal mode in which the loads normally consume power, the rectification/smoothing unit 105 outputs a plurality of output voltages, and if one of the output voltages is applied as a supply voltage of the microcomputer 114, a positive voltage is applied at the output terminal of the diode DD7 connected with the output terminal of the rectification/smoothing unit 109, so that the diode DD7 does not forwardly conduct. Therefore, the regulated voltage outputted from the rectification/smoothing unit 109 is not applied to the microcomputer 114. The microcomputer 114 receives a supply voltage from the rectification/smoothing unit 105 and controls the entire operations of the system based on a timing signal generated by the timer 112 for thereby controlling the power state of the loads.
When the microcomputer 114 checks the power consumption state of the loads, if it is judged that the loads do not consume much power, the microcomputer 114 controls the system and changes the normal mode into a power saving mode.
In the power saving mode, the microcomputer 114 outputs an ON/OFF control signal which controls the switching device Q1 at a low frequency by controlling the driving controller 106 via the signal feedback unit 107 so that power corresponding to the lower power consumption of the loads is outputted. Namely, when the microcomputer 114 increases the ON/OFF operation time of the switching device Q1, the levels of the voltages induced in the secondary windings of the main transformer 104 are decreased. Therefore, the rectification smoothing unit 105 which receives the lower voltage levels supplies DC voltages corresponding to the power consumed by the loads.
If the microcomputer 114 judges that the loads do not consume power, the operation mode of the switched mode power supply is changed to a standby mode. In the standby mode, the microcomputer 114 controls the driving controller 106 via the signal feed back unit 107, so that the switching device Ql is not operated, and the inductive voltages are not generated in the secondary windings of the main transformer 104. Therefore, the rectification/smoothing unit 105 does not output any voltages. In the standby mode, only the auxiliary power supply circuit continues to operates. When the power supply assumes a standby mode in accordance with the control of the microcomputer 114, the regulated voltage outputted from the rectification/smoothing unit 109 of the auxiliary power supply circuit is applied as a supply voltage to the microcomputer 114 via the diode DD7. Here, when the power supply operates in the normal mode or the power saving mode, the backup unit 113 receives an output voltage from the rectification smoothing unit 105 or 109 and charges to a certain voltage. Even in the standby mode, if the power is OFF, the charged voltage is supplied to the microcomputer 114, so that the microcomputer 114 continuously operates.
In the conventional switched mode power supply, there are provided both the main power supply circuit which supplies electric power to the loads in the normal mode, and the auxiliary power supply circuit which supplies a minimum power when the system operates in the standby mode in which the loads do not consume power. Therefore, the system becomes bulky and complicated. In the normal mode in which the loads normally consume power, since the main power supply circuit and the auxiliary power supply circuit are both operated at the same time, excessive noise is generated at the switching devices Q1 and Q2. In addition, since the switching devices Q1 and Q2 are continuously operated, as time elapses, the reliability of the switching devices Q1 and Q2 is decreased, so that the power consumption of the switched mode power supply is increased. Therefore, the switched mode power supply in accordance with the conventional art is not widely used for an electric appliance. Namely, the above-described switched mode power supply is used only for an expensive apparatus.
As another example of a conventional switched mode power supply, there is known a power supply in which the power outputted from the secondary winding of a transformer is controlled in the standby mode.
However, the above-described example also becomes complicated, since the system must include a higher number of circuit parts. Additionally the system is bulky, the reliability of the system is decreased, and also the fabrication cost is increased.
In a switched mode power supply for turning off the unnecessary power supply circuit in a standby mode, there is a limit on decreasing the power consumption since the switched mode power supply which uses power outputted from the auxiliary power supply circuit in the standby mode must necessarily include a certain number of parts. Therefore, the power supply becomes complicated, and the reliability of the circuit is also decreased.
A power supply capable of disconnecting from the AC power source by using a mechanical switch, in the view of decreasing the power consumption, offers advantages. However, since the AC power source turn on/off switch must be operated mechanically whenever the loads require power, the power supply is very inconvenient to use. In addition, when the AC power source on/off switch is turned off, for example, an apparatus such as a VCR can not record a reserved TV program or can not perform other functions.